Field of the Invention
The invention relates to a device for measuring range.
The prior art Meinke, Gundlach, Taschenbuch der Hochfrequenztechnik, 5th edition, Springer-Verlag, pages S3-S4 discloses a continuous-wave radar, which is also known as FM-CW radar. FM stands for frequency modulation and CW stands for continuous wave. A continuous-wave radar of this typo continuously transmits a radar signal whose frequency is varied continuously by the frequency modulation. The radar signal reflected by an object arrives at the radar sensor after the signal propagation time t.sub.L ##EQU1## where c=propagation velocity of the radar signal,
L=range of the continuous-wave radar from the object, PA1 .alpha.=df/dt=constant PA1 f.sub.o =base frequency.
and is compared there with the already present transmission frequency which has been varied in the meantime by the frequency modulation. The radar signal which, for example, has been transmitted at time t.sub.1 at the frequency f(t.sub.1), in, at the time of its return t.sub.2 =t.sub.1 +t.sub.L, compared with the transmitted signal with the frequency f (t.sub.2). The two signals are compared using a mixer. The frequency obtained at the output of the mixer is the mixing frequency .DELTA.f of the two signals: EQU .DELTA.f=f(t.sub.2)-f(t.sub.1)=f(t.sub.1 +t.sub.1)-f(t.sub.1)
The mixing frequency .DELTA.f is proportional to the range L between the object and the sensor. So that, for a given optimum object range L, the mixing frequency .DELTA.f remains constant during a time interval, it is necessary for the frequency modulation to be linear as a function of time during the relevant interval, i.e. the frequency increases or decreases linearly as a function of time: EQU f(t+.DELTA.t)=f.sub.0 +.alpha..multidot..DELTA.t
where
An example of a suitable modulating signal would be a sawtooth oscillation.
Complex evaluation of the mixing frequency .DELTA.f, that is to say of the FM Doppler signal, gives rise, depending on whether the instantaneous frequency is increasing or decreasing, to positive or negative distance-proportional frequencies which can be distinguished from velocity-proportional frequencies due to the object's motion.
Problems arise when measuring very distant objects on account of the limited coherence length of the transmitted signal. This means, even if the frequency modulation of the transmitted signal is exactly linear as a function of time, statistical phase and frequency fluctuations f.sub.x (t) are superimposed on the transmitted signal: EQU f(t+.DELTA.t)=f.sub.0 +.alpha..DELTA.t+f.sub.R (t)
Fluctuations of this type result from the phase noise of the transmission oscillator. This means that, between two instants, the phase of the received signal can deviate from the expected value by a particular value which is commensurately greater as the phase noise of the oscillator becomes stronger and the time between the two instants becomes longer. The result of this is that, depending on the phase noise of the signal source, there is a time period after which the phase difference fluctuates so greatly that a meaningful measured value can no longer be determined. The propagation length of the microwave signal (=radar signal) corresponding to this time period is referred to as the coherence length. In the came of object ranges for which the signal propagation length exceeds the coherence length, it becomes very difficult if not impossible to measure the range.
The problem can be solved by equipping the transmitter with a low-noise oscillator. This implies a largo coherence length. However, the production of an oscillator of this type requires the use of expensive components.
Range finders having FM-CW radar are also described in U.S. Pat. Nos. 4,205,316 and 4,360,812.